JFET TL072 based pre amp repair suggestions

[Mr.Ian]

To cut a long story short my Toft LBPA-5 had a channel imbalance and I had a couple of local "repair" guys look at the first managed to lift the tracking off the PCB and the second told me that it had components blown, it measured awfully and wasn't worth repairing - I took that to mean he couldn't fix it.

To be honest both of them are more biased towards valves so a TL072 based pre amp might have been too big an ask for them.

I haven't had much luck finding a replacement LBPA-5 so am thinking maybe try another repair.

The pot was out of trim at low volumes and this I think has been addressed by changing resistors in the output circuit. Three pots later and it was still out at low volumes.

After lifting the tracking to the volume off the board - the person responsible hard wired the pot but it doesn't work properly so I guess the taps have been taken from the wrong places.

Nightmare.

Any suggestions for someone who has good experience with JFET based amps who might be able to fix it - the repair isn't time critical, East Midlands area would be great - I am close to J28 on M1

[Firebottle]

If you are willing to travel to the West Midlands in a couple of weeks time........

:)

[Mr.Ian]

Hi Alan

yes travel to West Mids shouldn't be a problem, about most days.

Ian

[Light Dependant Resistor]

Hi
Looks like Alan is assisting. It is unusual for an opamp to fail, more likely
a voltage rail issue, failed resistor or mistake in prior repair.

The TL072 is still a good sounding device but there are differences in each brand.
The ST brand device has difference in internal circuit values and is IMO the better
brand to go with, ST appear to have put quite a bit of effort into it,that is if the
TL072 op amp needs replacement.

Arguably the best audio dual opamp is the LM4562 www.ti.com/lit/ds/symlink/lm4562.pdf
and is pin for pin replacement.

If you get Alan to fit some turned pin sockets these will enable assessment of
one vs the other, if you ever need to do so.

Cheers / Chris

[Mr.Ian]

Many thanks for everyone's interest

The full circuit has, from memory, about 15 op amps - each in its own push in/pull out socket and each controlling a different function. eg input, phase reverse, mute, tape monitor etc

http://i665.photobucket.com/albums/vv15/Mr_Ian/Toft%20Audio/DSC00197.jpg (http://s665.photobucket.com/user/Mr_Ian/media/Toft%20Audio/DSC00197.jpg.html)

My original thought was that the volume pot had worn out but now I am less convinced; yes the pot is far from balanced at low volumes but with 3 being the same I wonder if something else may be the real problem

The first "engineer" said the pot controlled feedback levels rather than operating on the volume direct so some other problem such as voltage rails or resistor values as suggested ????

It all well beyond me ...........